Protective coating (alborizing)



Aug., 4, Q. R, E, LOWE lg@ PROTECTIVE COATING (ALBORIZING) Filed sept. 15, 1927 Patented Aug. 4, 1931 'l UNITED STATES -PJrEN'r oFFlcF.

RUSSELL EDLUND LOWE, OF NEW YORK, N. Y., ASSIGNOB TO DOHERTY RESEARCH COMPANY, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE PROTECTIVE COATING (ALBORIZING) Application led September 15, 1927.

It has long been an objective ofthose engaged in engineering to make iron and steel articles capable of withstanding comparatively high temperatures without scaling 5 while retaining the full strength and heat conductivity of the ferrous metal. Articles such as pipe stills, recuperators and the like are required to transmit heat while subjected to high temperatures under oxidizing conditions, often under high pressures. If the tubes of such devices are oxidized to form a scale the fraction of an inch thick, the strength of the tubes is materially weakened and they must be replaced at a considerable expense. It has been proposed to coat iron and steel with coatings of various materials with a view to preventing scaling of the iron or steel. Among other materials, aluminum has beenproposed as a protective coating for this purpose. The yso-calle'd calorized articles are composed of ferrous bodies having a ferro-aluminum coating formed by heating the iron or steel articles in powdered aluminum under a hydrogen atmosphere and at a considerable temperature. However, in order that the calorizing process be effective in reventing scaling, so much aluminum is a oyed wit the iron or steel body of the article as to weaken it materially when exposed to high temperature. Moreover, the calorizing process has not been worked out as to its application to articles vof large size or of unwieldy character.

It has been proposed also to `use thin coatings of aluminum metal on iron articles for decorative purposes and for lprotection against rust at ordinary temperatures. The thin aluminum coatings used for the purposes just mentioned, however, have not been of a character to resist more than a moderate heat or to protect the iron or steel beneath from oxidation.

It is one object of the present invention to provide a ferrous article lhaving an aluminum coating and capable of both Withstanding an oxidizing atmosphere at high temperatures without scaling and withstanding high pressures.

Another object of the invention is to pro- Serial No. 219,660.

vide a method of making the aluminum coated article.

Further yobjects and advantages of the present invention will be apparent to those skilled in the ymechanical and chemical arts from the following description taken with the accompanying drawings in which Fig. 1 illustrates an article embodying the present invention; z

Fig. 2 is` a detail view illustrating a portion of the article of Fi 1 as it appears under a magnification o two hundred diameters.

In the drawings, 10 is a tube of irouor steel of the kind used in boilers, pipe stills, recuperators and/or like structures. The outer surface of tube 10 is covered by a relatively Ithin but tough imperforate glazed layer or coating 12 inert to oxygen at 1400.

high temperature conditions. Said apertures cause the ferrous bod of an article to be attacked by oxygen. he coating 12 is preferably about .002 inch thick, the thickness being taken as an average over the entire coating. Also, the coating 12 should be alloyed to the body of article 10 in order to prevent scaling or aking of coating 12.

The thickness of the protective coating 12, may, however, range from .001 inch to .02 inch within the presentl invention, although no practical advantage has been found in employing coatings over .002 inch thick. Coatings avera ing much less than .O02 in thickness will e apt to prove imperfect` through the presence of pinholes, while heavier coatings may blister in the process of formation. An additional disdrying.

advantage inherent to excessively heavy coatings, even if the same have been successfully applied, lies in their -tendency towards disruption through dilnunition of their elasticity and the fact that there exists a difference in co-eflicient of expansion between the coating and unaffected steel. The thickness of coating 12 should be superficial with respect to the Wall of the ferrous portion of article l0 in order that the ferrous Wall or body retain its original strength. The coating 12 has a dull blue-gray color due to a slag Glaze 16 on its outer surface and is coniposed of slag and alloy, the slag occurring principally as separate particles in the alloy. In the drawin's, the slag particles in coating 12 are in( icated by dots 18 in Fig. 2, the alloy being indicated at 20. The outer portion of the coating 12 is high in slag andv low in metal or alloy and t ie percentage composition of coatinfr 12 varies 1n passing inwardly theretliroug i to an inner surface which is low in slag and high in metal. The slag in coating 12 is composed of alumina and iron oxide combined with borax. The alloy in coating 12 is an ironaluminum alloy and is richest in iron at the line or surface of juncture 14.

The article according to the present invent-ion and above described is formed preferably by the following process which also forms part of the resent invention:

The ferrous sur ace to be protected is thoroughl cleaned, usually by pickling, and

protective coating is then formed on the dry surface. For this purpose preferably a liquid carrier is prepared containing aluminum powder or Hake in suspension. The-carrier liquid, if used, must have sufficient fluidity to spread easily, but it must have suHicient body to carry the metallicaluminum flake and a Hux in suspension without agglomeration so that an even thickness of aluminum coating may be formed. The mixture must not, however, be so Huid that it spreads too thin and thereby deposits too little aluminum per unit area of surface to be protected, or so thick that too much aluminum is de osited er unit area of ferrous surface. he carrier must also volatilize under heat without excessive formation of undesirable material, such as carbon, and Without peeling or blistering. It has been found that a suitable liquid carrier for use in the present process is one formed by mixing four parts of rectified spirits of turpentine with one part acidfree s ar varnish. Tur entineand spar varnis are both well nown articles of commerce. The spar-varnish of commerce While of varying composition according to the maker, yields an elastic non-perforate dense coating or glaze when dry and imparts these desirable characteristics to the deposit formed by the liquid when dried on a surface to be coated according to the present invention. Benzene (benzol), and spar varnish, cellulose and a cellulose solvent, and a solution of rubber 1n a rubber solvent are exam les of other liquids that may be used in t 1e present process as carriers for aluminum flake in place of the said turpentine-varnish mixture. Am l acetate is an example of a suitable cel ulose solvent and acetone, acetone and ether, and benzene are exam les of rubber solvents which ma be use in making cellulose or rubber so utions for use as carrier liquids when racticing the present invention. The ingre lents of the carrier must be substantially free from water.

`The aluminum for forming the protective coating is incorporated into the liquid carrier, preferably in the form of aluminum Hake. It has been found that the characteristics of the coating formed depend in lar e measure upon the size of the aluminum Ha e which is used. If the very fine sizes of aluminum powder or Hake are used, the coating produced is unsatisfactory; either being too thin or containing sq much alumina (oxide of aluminum) as to form holes in the coating. It therefore fails under high temperature. On the other hand, if the flake is too course, it settles out of the carrier liquid and does not make a uniform coatin #46 commercial aluminum flake is satisfactory. The aluminum Hake is incorporated into the liquid carrier and with it is incorported a Hux material. The material used as Hux must have certain characteristics. It must be substantially dry under the conditions of the process. If Water is driven off during the operation of coating, the coating is blistered and its aluminum content more or less completely oxidized and no protection to the underlying metal is afforded. Also the. Hux material must melt at a low enoufrh temperature to protect the aluminum Hake from oxidation While the Hake is alloying with the ferrous body. The Hux material must have a good solvent action on iron oxide and a solvent action, although a less one, on the aluminum oxide. The only known satisfactory Hux for use in the present process is finely divided anhydrous fused borax. Ordinary hydrous borax is not suitable for the reason that it causes blisters in the coating.

lVhen the aluminum Hake and the flux have been incorporated into the liquid carrier, (assuming a carrier is used), the mixture is applied evenly to the cleaned surface of the ferrous article to have the protective coating formed thereon. The mixture is allowed to dry and resulting layer of dried liquid carrier, aluminum Hake and Hux is then heated quickly to the melting point of the flux and the heating continued until the desired adherent protective coating is formed. A high temperature Hame may be used lfor heatin the dried mixture as just described. During the heating to form the protective coating, the dense nonperforate elastic glaze formed by the dried liquid carrier serves to partly protect the finely divided aluminum from oxidation until the Hux melts, the melted Hux ythen partly protecting the aluminum Hake from oxidation. The organic carrier .liquid is completely driven ol" or burned by the heating of the dried mixture in the formation of the desired protective coating.

By way of example, a specific manner of practising the process for producing the protective coating on ferrous materials according to the present invention is as follows:

A convenient quantity of aluminum for forming the basis of a batch of coating material is first taken. Fourteen grams of No. 46 chemically pure aluminum Hake is a suitable amount and character of material for this purpose. A typical screen analysis of No. 46 aluminum Hake shows approximately that two-tenths of one percent of #46 Hake is retained on a screen having 100 meshes to the inch; of the material passing through 100 mesh screen 1.6% is retained on a 120 mesh screen; of the material passing 120 mesh screen 21.2% is retained on a screen having 140 mesh per square inch; of the material passing the 140 mesh screen, 6.5% is retained on a 160 mesh screen; of the material passing the 160 mesh screen, 1.8% is retained on the 180 mesh screen; of the material passing the 180 mesh screen 23.5% is retained on the 200 mesh screen; and 45.2% passes the 200 mesh screen. It will be noted that greater than 50% of the material is coarser than 200 mesh.L The invention is not limited to No. 46 Hake, or to Hake of the same Hneness. It has been found that #20 Hake is too coarse and #99 Hake or powder is too line for best results. Sizes between Nos. 20 and 99 Hake may be usefully employed. The reasons whythe Hake or powder used must lie within certain limits of fineness appear above. With the 14 grams of Hake are incorporated preferably 14 grams of fused anhydrous borax, 200 mesh or finer, and the whole then mixed with 8() cubic centimeters of rectified spirits of turpentine and 20 cubic centimeters of acid-free spar varnish. After the ferrous surface has been prepared as mentioned above, the mixture containing the ingredients substantially as just stated, or their equivalents, is then Howed over the surface or otherwise a plied thereto in such manner as to unifbrmly and thoroughl coat the entire surface. The mixture, w en of the proper consistency has a distinctly tacky nature, said-consistency materially aiding in suspending the borax and the metal Hake and in causing adherence of the mixture to the metallic surface to be coated. A batch of the mixture containing 14.- grams of 46# aluminum Hake, 14 grams of finely divided fused borax, 80 cc of rectified spirits of turpentine and 20 cc acid-free spar varnish coats when used in the present process about 3 sq. ft. of surface. The dried deposit and also the finished protective coating therefore contain about 4.0 grams of aluminum per sq. ft. After the mixture has been applied to the article, it is then set aside to dry, preferably at atmospheric temperature, until the mixture is distinctly hard. The

hardening of the liquid mixture has beenv found to take about an hour under ordinary conditions. After the mixture has hardened the external layer formed on the article is about .002 inches in thickness. The article is then heated to drive 0H' the liquid vehicle from the mixture, to fuse the Hux and aluminum, and to form the coating 12. Preferably the heating of the article for the purposes just mentioned is performed by flashing with an Oxy-acetylene torch using a broad Hame about 1 inch in diameter.

until the section under heat treatment assumes a dull blue gray color. Actual temperature attained during Hashing=l400 1550o F. In the language of the operatives, the coating is then properly cooked. The desired color is acquired in a very short time and therefore the torch may be kept moving slowly and the Hashing made continuous. It is necessary to the formation of a satisfactory coating 12 that the temperature of the Hux be brought to its melting point Very quickly so that it will fuse on the aluminum grains and protect them from over oxidation. It has been found, for instance, that the Hame of an ordinary Bunsen burner does not melt the Hux quickly enough for good results when used in the open air. The result of heating too slowly is to cause so much of the aluminum Hake or powder to oxidize before it melts and alloys with the ferrous surface that the resulting coating contains holes and is of a friable and blistery character unless the steel being treated is of very thin section. After the Hux has fused, it not only protects the aluminum, but dissolves the iron and aluminum oxides on the surfaces in contact and so promotes the alloying of the aluminum to the ferrous body. Of course, the coating 12 does not adhere properly to the body 10 unless the aluminum alloys with the body 10, at least at the surface of the body. The aluminum alloys with the iron to the desired degree under the conditions mentioned above.

The articles according to the present invent-ion retain their full strength under very severe heat conditions. This is borne out by a comparative experiment using calorized tubes and tubes according to the present invention. The calorized tubes and i QL those treated by the present44 process were of the same original dimensions and ma? maintained in an oxidizingatmosphere at a temperature of 1200 F. The calorized tubes under the conditions mentioned softened and swelled after three days. The tubes according to the present invention, however were subjected to an vinternal air pressure of 1000 lbs. while surrounded by an oxidizing atmosphere for six months at temperatures ranging between 1000 F. and 1500o F., but without showing signs of distress. The coating, however, began to blacken when held for a time at temperatures above 1400o F. `While an article according to the present invention may be held at 1400" F. for some time without showing signs of oxidation, the ordinary upper working limit of temperature for articles according to the present invention over periods of time as long as six months -is 12o0 F.

It will be understood that the tubes aecording to the present invention which were tested as above described by the present process both internally and externally. It is one of the advantages of the present process that a protective coating may be formed on both the inner and outer surfaces of hollow articles of moderate thickness and at the same time. In order to form a protective coating on both inner and outer surfaces to an article it is necessary only to flow the liquid mixture of the character described in the above example over both surfaces to form a deposit containing the aluminum and flux. When the deposit is dried as above described it may then be flashed by the high temperature iiame, the fiame being applied to either surface as is most convenient, the resulting heating producing a protective coating according to the present invention on both surfaces simultaneously.

While it is preferred to use a liquid vehicle or carrier to apply the aluminum powder or iiake and borax iuX to the surface on which a protective coating according to the present invention is to be formed, the use of a liquid vehicle or carrier may be omitted. A suitable-method of forming a protective coating accordingy to the present invention when no liquid carrier is employed is as follows: The ferrous surface to be coated is cleaned and then an even layer of aluminum powder or flake of suitable size is sifted or otherwise spread on said surface. Said layer should contain a suitable Weight of aluminum per square ft. of surface. As above indicated, .l5 oz. of aluminum per sq. ft. of surface to be coated is a suitable weight of aluminum, but the invention is not limited to this. Powdered anhydrous borax may then be sifted or spread evenly in other Ways over the layer of aluminum powder or flake. The borax powder is used in suteient quantity to cover the aluminum and to protect it in large measure v against oxidation during the subsequent heating period. A weight of borax powder equal per sq. ft. to that of the aluminum is a suitable amount of borax, but the invention is not limited to this. After the aluminum and borax powders have been applied to the surface to be coated, the ferrous article is then heated quickly to melt the borax and to form a protective aluminum containing coating which alloys with the surface of the ferrous article. The heating of the article to be coated when the liquid carrier is omitted may be performed in an enameling furnace. Of course, a blast fiame cannot be applied directly to the surface carrying the powdered aluminum and borax since the blast would scatter the powders. In case of a thin sheet however, a blast flame, such that from an oxyacetylene lianie, can be applied to the underside of a sheet whose top horizontal surface carries a layer of powdered aluminum and powdered anhydrous borax to melt the borax and aluminum and to form the desi red protective coating on the upper surface of the sheet. Moreover, I find that, when desired, coatings of a thickness up to .02 inches can` be built up by spraying, provided the metal has been preheated to and is mailitained at such temperature that the liquid vehicle in which the aluminum flake is suspended is rapidly volatilized. It will be evident also from the foregoing that I am not limited to the use of the liquid carriers mentioned above, other liquids having been used as carriers in forming aluminum coatings for decorative and like purposes.

As above indicated, it is preferred to use aluminum powder or iake when operating according to the present invention at the rate of approximately .15 oz. of aluminum per sq. ft. of surface to be coated. A coating according to the present invention and having .l5 ounces aluminum per sq. ft. protects against scaling as above mentioned. However, useful results as to protection from scaling are obtained when using as little as .05 'ounces of aluminum per sq. ft. and as much as .25 ounces per sq. ft. of surface to be coated. Moreover, the amount of borax used need not be equal to the amount per sq. ft. of surface to be coated, it being practicable within the present invention to protect against scaling when varying the ratio between the borax and the aluminum from as much as two parts of borax to one of aluminum to as little as one of borax to two of aluminum; provided, however, the amount of borax does not fall below .05 ounces per sq. ft. when making yan anti-scaling coating.

ln the foregoing description, the resent invention is disclosed as applied to iron or ferrous articles. lt will be understood however that the invention is not limited to protecting the surfaces of articles composed essentially of iron, but maybe applied likewise to alloys containing considerable amounts or percentages of iron, such for instance, as nickel-steel, chrome-steel, man anese-steel, vanadium-steel, and molyb enum-steel. v

Also it will be evident from the foregoing that one aspect of the present invention involves the discovery ofthe advantages Howing from the use of anhydrous fused borax as a Hux when forming aluminum contain ing coatings on surfaces of ferrous articles and that while of particular usefulness in protecting iron against high temperatures, the coatings formed when using a Hux of fused anhydrous borax may be used also for decorative andrust preventive purposes. When aluminum containing coatings are to be formed for decorative and/or rust preventive purposes only, the quantity limits stated above do not apply.

Having thus described my invention, I claim: l

1. The process of formin a protective coating on. a ferrous sur ace includin covering the ferrous surface to be protected witha mixture of aluminum Hake of a size between those of Nos. 20 and 99 and finely divided anhydrous fused borax in a liquid vehicle, said vehicle holding the Hake and borax in suspension without agglomeration while forming a tacky mixture, drying said mixture, and heating said dried mixture rapidly enough to coat the aluminum particles with fused borax before a material Vpercentage of the aluminum Hake has been ferrous body in a thin layer forming an :im-v

perforate coatin having a glazed surface.

2. The method of forming a rotective coating on a ferrous article comprlsing covering the surface of said article to be coated with a uniform deposit containing between .05 oz. and 1.25 oz. of divided aluminum per square foot of said surface, said de osit containing also anhydrous borax, and a loying said aluminum with said, surface by heat' under conditions deterent to the oxidation of said aluminum.

3. The method of formin a protective coating on a ferrous artic e comprising cleaning the surface to be coated, -covering said surface with a uniform layer containing approximately .15 oz. of finely divided aluminum per square foot and an a proximately equal amount of finely divide anhydrous borax, and alloying said aluminum with said surface under conditions deterent to the oxidation of said aluminum. 4. The method of forming a protective coating on a ferrous article comprising cleaning the surface lto be coated, Howing onto said surface a mixture of a carrier liquid holding in suspension Hnely divided aluminum and fiuel divided anhydrous borax and drying sald liquid on said surface to form a deposit containing uniform amounts of aluminum and borax per square foot of surface, and heating said deposit under conditions deterrent to oxidation of said aluminum to alloy' said aluminum to said surface and to form a uniform coating thereon.

5. rlhe process of forming a protective coating on a ferrous surface including covering said surface with a mixture-of aluminum Hake of a size between those of Nos. 20 and 99 and finely divided anhydrous fused borax in a liquid vehicle, said vehicle holding said flake and said borax in suspension withoutagglomeration While forming a tacky mixture and containing turpentine and spar varnish, drying said mixture on said surface to form a uniform deposit, and heating said deposit rapidly enough to coat y,the aluminumparticles with fused borax before a material percentage of the aluminum Hake has been converted into oxide, to drive oHt' the dried vehicle and to alloy the aluminum to said surface in a thin imperforate layer.

6. Ina method of forming an aluminum containing coating on the surface of a ferrous article, the step including heating a| layer of aluminum on said surface in the presence of finely divided anhydrous fused borax.

7. In the manufacture of a coated ferrous article, the improvement comprising the steps of covering the ferrous surface to be' coated with a uniform layer of aluminum Hake and finely divided fused borax, said layer containing between .05 ounces and .15 ounces of alumlnum per square foot and between .05 ounces and .15 ounces of borax per square foot, and quickly heating said layer to 1400 F. under conditions deterrent to oxydation to alloy said aluminum to said surface and to form an aluminum containing coating inert to oxygen at 1200. F. over long periods.

In testimony whereof I afiix my signa-l RUSSELL EDMUN D LOWE.

ture. 

